CS 356: Computer Network Architectures Lecture 22: Internet Quality - PowerPoint PPT Presentation

CS 356: Computer Network Architectures Lecture 22: Internet Quality of Service [PD] Chapter 6.5 Xiaowei Yang xwy@cs.duke.edu

Overview • Network Resource Allocation • Congestion Avoidance • Why QoS? – Architectural considerations • Approaches to QoS – Fine-grained: Integrated services • RSVP – Coarse-grained: • Differentiated services • Next lecture 2

Internet Quality of Service

Motivation • Internet currently provides one single class of “best-effort” service – No assurance about delivery • Many existing applications are elastic – Tolerate delays and losses – Can adapt to congestion • “Real-time” applications may be inelastic 4

Inelastic Applications • Continuous media applications – Lower and upper limit on acceptable performance – Below which video and audio are not intelligible – Internet telephones, teleconferencing with high delay (200 - 300ms) impair human interactions • Hard real-time applications – Require hard limits on performance – E.g., industrial control applications • Internet surgery 5

Design question #1: Why a New Service Model? • What is the basic objective of network design? – Maximize total bandwidth? Minimize latency? Maximize ISP’s revenues? – the designer’s choice: Maximize social welfare: the total utility given to users (why not profit?) • What does utility vs. bandwidth look like? – Must be non-decreasing function – Shape depends on application 6

Utility Curve Shapes U Elastic U Hard real-time BW BW Delay-adaptive U • Stay to the right and you are fine for all curves BW 7

Playback Applications • Sample signal à packetize à transmit à buffer à playback – Fits most multimedia applications • Performance concern: – Jitter: variation in end-to-end delay • Delay = fixed + variable = (propagation + packetization) + queuing • Solution: – Playback point – delay introduced by buffer to hide network jitter 8

Characteristics of Playback Applications • In general lower delay is preferable • Doesn’t matter when packet arrives as long as it is before playback point • Network guarantees (e.g., bound on jitter) would make it easier to set playback point • Applications can tolerate some loss 10

Applications Variations • Rigid and adaptive applications – Delay adaptive • Rigid: set fixed playback point • Adaptive: adapt playback point – E.g. Shortening silence for voice applications – Rate adaptive • Loss tolerant and intolerant applications • Four combinations 11

Applications Variations Really only two classes of applications 1) Intolerant and rigid 2) Tolerant and adaptive Other combinations make little sense 3) Intolerant and adaptive - Cannot adapt without interruption 4) Tolerant and rigid - Missed opportunity to improve delay 13

Design question 2: How to maximize V = ∑ U(s i ) • Choice #1: add more pipes • Choice #2: fix the bandwidth but offer different services – Q: can differentiated services improve V?

If all users’ utility functions are elastic U Elastic Does equal allocation of bandwidth maximize total utility? Bandwidth • ∑ s i = B • Max ∑ U(s i ) 15

Design question: is Admission Control needed? • If U(bandwidth) is concave U Elastic à elastic applications – Incremental utility is decreasing with increasing bandwidth BW • U(x) = log(x p) • V = nlog(B/n) p = logB p n 1-p – Is always advantageous to have more flows with lower bandwidth • No need of admission control; This is why the Internet works! And fairness makes sense 16

Utility Curves – Inelastic traffic Delay-adaptive U U Hard real-time BW BW Does equal allocation of bandwidth maximize total utility? 17

Is Admission Control needed? • If U is convex à inelastic Delay-adaptive U applications – U(number of flows) is no longer monotonically increasing – Need admission control to BW maximize total utility • Admission control à deciding when the addition of new people would result in reduction of utility – Basically avoids overload 18

Incentives • Who should be given what service? – Users have incentives to cheat – Pricing seems to be a reasonable choice – But usage-based charging may not be well received by users

Over provisioning • Pros: simple • Cons – Not cost effective – Bursty traffic leads to a high peak/average ratio • E.g., normal users versus leading edge users – It might be easier to block heavy users

Comments • End-to-end QoS has not happened • Why? • Can you think of any mechanism to make it happen?

Approaches to QoS • Fine-grained: – Integrated services • RSVP • Coarse-grained: – Differentiated services 22

Components of Integrated Services 1. Service classes What does the network promise? 2. Service interface How does the application describe what it wants? 3. Establishing the guarantee How is the promise communicated to/from the network How is admission of new applications controlled? 4. Packet scheduling How does the network meet promises? 23

1. Service classes What kind of promises/services should network offer? Depends on the characteristics of the applications that will use the network … . 24

Service classes • Guaranteed service – For intolerant and rigid applications – Fixed guarantee, network meets commitment as long as clients send at match traffic agreement • Controlled load service – For tolerant and adaptive applications – Emulate lightly loaded networks • Datagram/best effort service – Networks do not introduce loss or delay unnecessarily 25

Components of Integrated Services 1. Type of commitment What does the network promise? 2. Service interface How does the application describe what it wants? 3. Establishing the guarantee How is the promise communicated to/from the network How is admission of new applications controlled? 4. Packet scheduling How does the network meet promises? 26

Service interfaces • Flowspecs – TSpec: a flow’s traffic characteristics • Difficult: bandwidth varies – RSpec: the service requested from the network • Service dependent – E.g. controlled load

A Token Bucket Filter Tokens enter bucket Operation: at rate r – If bucket fills, tokens are discarded – Sending a packet of size P Bucket depth b : uses P tokens capacity of bucket – If bucket has P tokens, packet sent at max rate, else must wait for tokens to accumulate 28

Token Bucket Operations Tokens Tokens Tokens Overflow Packet Packet Not enough tokens Enough tokens à à wait for tokens to packet goes through, accumulate tokens removed 29

Token Bucket Characteristics • In the long run, rate is limited to r • In the short run, a burst of size b can be sent • Amount of traffic entering at interval T is bounded by: – Traffic = b + r*T • Information useful to admission algorithm 30

Token Bucket Specs BW Flow B 2 Flow A: r = 1 MBps, B=1 byte 1 Flow A Flow B: r = 1 MBps, B=1MB 1 2 3 Time 31

TSpec • TokenBucketRate • TokenBucketSize • PeakRate • MinimumPolicedUnit • MaximumPacketSize

Service Interfaces: RSpec • Guaranteed Traffic – TokenRate and DelayVariation – Or DelayVariation and Latency • Controlled load – Type of service 33

Components of Integrated Services 1. Type of commitment What does the network promise? 2. Service interface How does the application describe what it wants? 3. Establishing the guarantee How is the promise communicated to/from the network How is admission of new applications controlled? 4. Packet scheduling How does the network meet promises? 34

RSVP Goals • Used on connectionless networks – Robust – Should not replicate routing functionality – Should co-exist with route changes • Support for multicast • Modular design – should be generic “signaling” protocol • Approaches – Receiver-oriented – Soft-state 35

RSVP Service Model • Make reservations for simplex data streams • Receiver decides whether to make reservation • Control msgs in IP datagrams (proto #46) • PATH/RESV sent periodically to refresh soft state 36

PATH Messages • PATH messages carry sender’s Tspec – Token bucket parameters • Routers note the direction PATH messages arrived and set up reverse path to sender • Receivers send RESV messages that follow reverse path and setup reservations • If reservation cannot be made, user gets an error 37

RESV Messages • Forwarded via reverse path of PATH • A receiver sends RESV messages – TSpec from the sender – Rspec 38

Admission control • Router performs admission control and reserves resources – If request rejected, send error message to receiver – Guaranteed service: a yes/no based on available bandwidth – Controlled load: heuristics • If delay has not exceeded the bound last time after admitting a similar flow, let it in

Soft State to Adapt to Routing Changes • Problems: Routing protocol makes routing changes • Solution: – PATH and RESV messages sent periodically – Non-refreshed state times out automatically • Ex: a link fails. How is a new reservation established? 40

Merging multicast reservations A requests a delay < 100ms B requests a delay < 200ms